I guess by FX trading you mean “many positions on currency pairs that I do not intend to keep open more than 6 months” => This would break one the criteria that tax office uses to decide if you are a “professional” or not. if they deem you professional, then all of your capital gains (including your mustachian investments in ETFs) will be considered as taxable income, and thus taxed at your marginal tax rate…

This is not a sure thing, it is entirely in the discretion of the tax office if they declare you a professional trader or not. Technically all mustachians could get taxed this way once they retire as capital gains will make up most of their income.

if FX buy/sell under 6 months can cause all your capital gains being taxed at your marginal rate

than buying 200 EUR at Migros Exchange before going shopping to France, and then selling the remaining 50 EUR after coming back (or anytime before 6 months passes) might cause the Tax Office to classify one as a professional investor, and therefore tax all ETF gains at marginal rate.

This is a bit paranoid, but as a Pole, while dealing with the Tax Office I always assume the worst possible outcome very probable.

I can bet that most of us here could be qualified as professional investors in the event the Tax Office decides so.

“Show me the man, and I’ll show you the crime.”

If you are one of these people that never exchanged back the remaining foreign currency after coming back from holidays, maybe you:

tried buying BTC etc. and then changed your mind and sold it

sold an unwanted gift after Christmas

…

Is being qualified as a “professional investor” a lifetime “badge”, or per tax-year?
If one do not sell their ETFs in the accumulation phase (let’s assume there is no need to re-ballance by selling if one has VT only or 2 ETFs) - therefore how bad is being qualified as “professional investor”?

Technically all mustachians could get taxed this way once they retire as capital gains will make up most of their income.

At least at the beginning after reaching FIRE, you should not withdraw more than 4% of your portfolio, 2% of it being withdrawn via dividend, which is taxable income, not treated as capital gains. But indeed, if there was a market crash, you could easily generate more capital gains than dividend income.